This invention relates to radio location, as of mobile vehicles such as aircraft equipped with transponders and, more particularly, to improvements on the inventions described in U.S. Pat. No. 4,115,771.
The system shown in U.S. Pat. No. 4,115,771, the disclosure of which is hereby incorporated herein by reference, known as a Passive Secondary Surveillance Radar (PSSR), utilizes the internationally standardized interrogation signals broadcast at a frequency of 1030 MHz by the large, rotating directional antenna of a nearby SSR, usually situated on the grounds of an airport, and the transponder signals transmitted at a second frequency of 1090 MHz by the on-board transponder of aircraft operating within the system's range, to provide accurate flight tracks, aircraft speed, altitude, vertical rates, beacon code identity, and aircraft-to-aircraft spacing, all locked to a common time base. This data is available in real time and can be stored for later evaluation.
There are hundreds of SSRs in commission in the United States, all of which transmit interlaced altitude and identity interrogations at the 1030 MHz carrier frequency, and all receive replies on the 1090 MHz carrier frequency. All ICAO nations, about 160, have also standardized on SSR. Many regions, particularly where air traffic is dense, are covered by the overlapping service areas of multiple SSR radars. An aircraft at en route altitude often responds to many radars, some as distant as 200 miles or more. Shared common carrier frequency operation of overlapping radars is made possible by assigning different beam rotation periods and different interrogation repetition patterns (rates and/or sequences) to different radars. Each radar is thus enabled to identify and process replies to its own interrogations, rejecting replies elicited by other radars. A PSSR can also segregate transponder replies associating them with individual SSRs.
The PSSR system is usually sited near but not on the airport grounds, typically a mile or two from an airport surveillance radar-SSR, and includes two antennas: a fixed, directional high gain 1030 MHz antenna aimed toward the selected SSR antenna for receiving the interrogation signals transmitted by that SSR. A 1090 MHz omnidirectional antenna is used for receiving transponder signals from nearby aircraft. A PSSR may also be associated with an Air Route Surveillance Radar (ARSR) not located on airports, but transmitting similar SSR signals for en route high altitude flights. The basic system can accurately track as many as thirty or more aircraft simultaneously in real time, and has been used in conjunction with a noise monitoring system for correlating noise events and flight tracks with aircraft identification with great accuracy, enabling a Noise Abatement Officer to respond to noise complaints, to assist in implementing noise-based restrictions, to document violations of flight corridors, and to monitor each take-off or landing operation. Multiple strategically located microphones measure noise levels of passing aircraft which are correlated with the PSSR aircraft tracks. For noise monitoring and abatement around an airport a range of about fifteen miles from the airport is acceptable and can be achieved with a simple omnidirectional antenna, consisting of little more than a dipole mounted on a flat plane, for receiving the 1090 MHz airborne transponder signals.
Among other possible applications of the PSSR system is the monitoring, by an airline's dispatch office, of its flights into a particular high-density airport so as to know the exact arrival times of its aircraft and thereby enable scheduling of gasoline trucks, food supplies, gate assignments and other significant dispatch operations essential to efficient operation and profit of that airline. Such monitoring would be done privately, independent of the Federal Aviation Administration (FAA) which, although possessing it, utilizes it for air traffic control purposes only, and normally will not provide an airline real-time, live arrival, position, identity and time data on its flights. To be useful for this purpose, however, the PSSR system must have a range of approximately fifty to one hundred miles, significantly greater than the 15-mile range of current systems. The object of the present invention is to extend the range of reception of the 1090 MHz receiving channel of the PSSR system disclosed in U.S. Pat. No. 4,115,771, by an amount to adapt it for this and other applications, without limiting the number of aircraft that can be tracked.